Ever since it was first reported that the contrast sensitivity for low spatial frequency sinusoidal gratings falls off with a slope approximating unity it has been suggested that this may represent gradient detection by a single spatial filter tuned to higher spatial frequencies. We have further investigated this possibility using aperiodic as well as periodic stimuli. Experiments are presented that involve manipulations of luminance, field size and spatial adaptation, and the results are consistent with a multiple filter model accounting for the detection of low spatial frequency (0.2-2 c/deg) periodic and aperiodic stimuli.